1. Field of the Invention
This invention relates to vertical cavity surface emitting lasers. More specifically, this invention relates to using higher order lasing modes in vertical cavity surface emitting lasers.
2. Discussion of the Related Art
Vertical cavity surface emitting lasers (VCSELs) represent a relatively new class of semiconductor lasers. While there are many variations of VCSELs, one common characteristic is that they emit light perpendicular to a wafer's surface. Advantageously, VCSELs can be formed from a wide range of material systems to produce specific characteristics. In particular, the various material systems can be tailored to produce different laser wavelengths, such as 1550 nm, 1310 nm, 850 nm, 670 nm, and so on. In general, VCSELs include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and electrical contacts.
FIG. 1 illustrates a typical VCSEL 10. As shown, an n-doped gallium arsenide (GaAs) substrate 12 has an n-type electrical contact 14. An n-doped lower mirror stack 16 (a DBR) is on the GaAs substrate 12, and an n-type graded-index lower spacer 18 is disposed over the lower mirror stack 16. An active region 20, usually having a number of quantum wells, is formed over the lower spacer 18. A p-type graded-index top spacer 22 is disposed over the active region 20, and a p-type top mirror stack 24 (another DBR) is disposed over the top spacer 22. Over the top mirror stack 24 is a p-type conduction layer 9, a p-type GaAs cap layer 8, and a metallic electrical contact 26.
Still referring to FIG. 1, the lower spacer 18 and the top spacer 22 separate the lower mirror stack 16 from the top mirror stack 24 such that an optical cavity is formed. As the optical cavity is resonant at specific wavelengths, the mirror separation is controlled to resonate at a predetermined wavelength (or at a multiple thereof). At least part of the top mirror stack 24 includes an insulating region 40 that is usually formed either by implanting protons into the top mirror stack 24 or by providing an oxide layer. The oxide layer can be formed, for example, in accordance with the teachings of U.S. Pat. No. 5,903,588, which is incorporated by reference. The insulating region 40 defines a conductive annular central opening 42. Thus, the central opening 42 forms an electrically conductive path through the insulating region 40.
In operation, an external bias causes an electrical current 21 to flow from the p-type electrical contact 26 toward the n-type electrical contact 14. The insulating region 40 and the conductive central opening 42 confine the current 21 such that it flows through the conductive central opening 42 into the active region 20. Some of the electrons in the current 21 are converted into photons in the active region 20. Those photons bounce back and forth (resonate) between the lower mirror stack 16 and the top mirror stack 24. While the lower mirror stack 16 and the top mirror stack 24 are very good reflectors, some of the photons leak out as light 23 that travels along an optical path. Still referring to FIG. 1, the light 23 passes through the p-type conduction layer 9, through the p-type GaAs cap layer 8, through an aperture 30 in the metallic electrical contact 26, and out of the surface of the vertical cavity surface emitting laser 10.
It should be understood that FIG. 1 illustrates a common VCSEL structure, and that numerous variations are possible. For example, the dopings can be changed (say, by providing a p-type substrate 12), different material systems can be used, operational details can be tuned for maximum performance, and additional structures, such as tunnel junctions, can be added.
While generally successful, VCSELs have problems. In particular, some applications require a high power laser source that produces a Gaussian-like beam intensity distribution in the far field. Unfortunately, VCSELs can and do support a large number of higher order lasing modes, conventionally designated in the LP approximation as m,l modes. In particular, modes with m>0, and with l=1, that produce “donut” or “necklace” or “flower-petal” beam intensity distributions that do not have the required far field properties. On the other hand, other applications, such as high-speed data communication over multi-mode optical fiber, often achieve best performance if the far field intensity is derived from a specific higher-order “necklace” mode.
One method of obtaining the desired Gaussian-like beam intensity distribution is to reduce the diameter of the active region. While this is successful in quenching higher order modes, a small diameter active region results in low output power, a large series resistance, and tight fabrication tolerances.
Fourier optics provides for phase-shift apertures that can filter and/or tailor the far-field intensity distributions of an incident field. Recent art has demonstrated the use of a phase filter in the aperture of a VCSEL that tailors a higher-order “necklace” incident mode into a far-field beam intensity distribution that appears Gaussian-like. For example, S. Shinada et al. in “Far Field Pattern Control of Single High Order Transverse Mode VCSEL with Micromachined Surface Relief” discloses the use of micromachined top surfaces layers that produce alternating π-phase shifts. The alternating π-phase shift layers can produce a reasonable Gaussian-like single-mode far field pattern.
While S. Shinada et al. disclose a technically interesting concept, their approach is less than optimal for many practical applications. For example, S. Shinada et al. use a focused ion beam to etch the top layer of the VCSEL to form the alternating π-phase shift layers. This can induce damage on the VCSEL itself. Furthermore, it is difficult to accurately control the phase shifts of the individual layers to achieve optically neutral slices. That is, a slice that does not modulate the reflection, as seen from the cavity, or the transmission, as seen from the far field.
Therefore, an improved Fourier optical system that produces a single-mode far field intensity distribution pattern from a higher order lasing VCSEL would be beneficial. Also beneficial would be a VCSEL having an optical phase filter that forms a Gaussian-like single mode far field pattern from a higher order lasing mode, with the optical phase filter implemented by a surface deposition of pie-slice filter elements that not only produce suitable optical path length differences to achieve the far field distribution, but that also protect the surface of the VCSEL. Even more beneficial would be a VCSEL having an optical phase filter that forms a Gaussian-like single mode far field beam intensity pattern from a selected higher order lasing mode, with the VCSEL implemented in a manner that promotes the selected higher order lasing mode. Also beneficial would be a VCSEL having an optical phase filter that forms a single mode far field beam intensity pattern from a selected higher order lasing mode, with optical structures that promote the selected higher order lasing mode, and with electrical current confinement that produces current flow into the active region that tends to enhance the selected higher order lasing mode. Further, in some circumstances it is beneficial to have a far field intensity resembling a specific higher-order mode pattern that is created by converting a lower-order lasing mode. Thus, the optical phase filter is preferably used in conjunction with other techniques that tend to pin the operating mode of the VCSEL to a selected mode.